1. Field of the Invention
Intake and exhaust performance of an internal combustion engine can be improved by increasing what is referred to as a "valve area". The term "valve area" refers to the total cross-sectional area occupied by intake and exhaust valves in a combustion chamber of each cylinder of the engine. For this reason, a multi-valve internal combustion engine is typically provided with more than one exhaust valve as well as a plurality of intake valves. To improve the output of such a multi-valve internal combustion engine, however, it is necessary for the intake and exhaust valves themselves to be small in size and light in weight so that they can follow actuating cam surfaces throughout a variety of speeds of rotation of the engine and, in particular, in a range of higher operating speeds.
2. Description of Related Art
One example of a three intake valve internal combustion engine is known from a Japanese patent application entitled "Four Cycle Engine," filed on Dec. 28, 1981, and published, as Japanese Unexamined Patent Publication No. 58-113512, on Jul. 6, 1983. This engine has the drawback that a reflux air stream is blown into a center intake port at the beginning of an intake stroke.
To eliminate the blowing of such a reflux air stream into the center intake port, leaving the center port closed at the beginning of an intake stroke was considered. In such an engine, however, another problem is caused, since gasoline, if discharged into the center intake port before or in an early stage of an intake stroke, will be trapped as fuel droplets in a throat of the center intake port. Such fuel droplets are, typically, difficult to sufficiently vaporize and spray into the combustion chamber. This results in deterioration of fuel combustion. Furthermore, in an engine having a spark plug installed at a center of each cylinder, fuel droplets stick to electrodes of the spark plug and smoke, so as to produce hydrocarbons (HC).